Hydraulic power steering systems assist drivers in turning maneuvers. Typically, torque to operate the power assist steering system is supplied by an engine. Thus, the engine control system must be able to compensate for the often sudden torque requirement from the steering system. Most engine control systems reject this type of torque disturbance by using engine speed as a feedback control variable. Because there must be some engine speed error for the engine controller to take action, fluctuations in engine speed arise at idle when the driver turns the steering wheel.
To minimize these engine speed fluctuations, power steering pressure switches and power steering pressure transducers have been used to measure the power steering pump pressure. The accessory disturbance torque is typically estimated as a function of the power steering pump pressure. The engine control system then uses the estimated torque to make corrections to the engine inputs. The engine control system reads the pressure switch or transducer, makes a calculation, and takes corrective action by adjusting an engine parameter, for example engine airflow, to compensate for the calculated torque disturbance. In this way it is possible to reduce the engine speed fluctuations. Such a system is disclosed in U.S. Pat. No. 5,097,808.
The inventors herein have recognized numerous disadvantages with the above approach. One disadvantage is performance limitations caused by the use of a pressure switch or a pressure transducer. The pressure measurement can not be used in other systems in the vehicle such as, for example, ride control systems. A second disadvantage is that because the pressure switch and transducer are measuring a pressure at the exact time the torque disturbance interacts with the engine, and because it takes a finite time for the engine control system to read the pressure switch or transducer, make a calculation, and take corrective action, there will be an engine speed fluctuation. Stated another way, by the time the engine control system has used the information from the pressure switch or transducer to adjust engine control parameters, the engine speed has already been affected by the accessory disturbance torque.